1. Field of the Invention
The present invention relates to the field of instrumentation amplifiers.
2. Prior Art
An Instrumentation Amplifier is often made up of 3 operational amplifiers (OpAmps). The first two amplifiers are buffer amplifiers. The third amplifier is an amplifier with a four-resistor bridge as a feedback network. This configuration has two main disadvantages: Firstly, the common-mode rejection ratio (CMRR) is limited by the unbalance of the resistive bridge. Secondly, the input voltage common-mode (CM) range cannot include the negative rail because of the overall feedback from the output to the input by the OpAmps (“Operational Amplifiers”, Johan Huijsing, Kluwer Academic Publishers).
Therefore, the current-feedback instrumentation amplifier is a better alternative. Its topology is shown in FIG. 1. It is excellently suited to allow the negative or positive supply rail voltage to be included into the input common-mode range (“Indirect current feedback instrumentation amplifier with a common-mode input range that includes the negative rail”, B. J. van den Dool et al., IEEE Journal of Solid State Circuits, Vol. 38, No. 7, July 1993, Pgs. 743–749). The reason is that the input signal and feedback signal are independently sensed by the voltage-to-current (V-I) converters G3 and G4. For instance, if these V-I converters are composed of identical differential P-channel pairs, the negative supply rail can be included. For obtaining a better accuracy and CMRR, the V-I converters can be each composed of two high-transconductance composite P-channel transistors with a degeneration resistor between the sources. This also improves the matching of the two identical transconductances G3 and G4 for better overall gain accuracy.
The instrumentation amplifier of FIG. 1 further consists of an output stage G1 and an intermediate stage G2. A nested Miller compensation with CM11, CM12, CM21, CM22 provides a preferred straight roll-off of the frequency characteristic.
To obtain low offset, choppers can be inserted in the signal path around the input stages, as shown in FIG. 2. With choppers, the offset can roughly be reduced by a factor 100–1000, from 10 mV to 100–10 μV. But there are several limitations. Firstly, a square wave at the chopper frequency of the size of the offset referred to the input will appear around the correct average signal value. To erase this square wave, a low-pass filter has to be placed after the instrumentation amplifier. This reduces the bandwidth of the instrumentation amplifier to below 0.1 (10%) of the chopper frequency. If the chopper frequency F1 is 10 kHz, the bandwidth will be reduced to several hundreds Hz.
Secondly, there are several effects that limit the offset reduction. One of them is an imperfect 50% duty cycle of the chopper frequency. Another is an unbalance of the charge injection in the choppers by the switching signal. Further, the initial offset will not fully be averaged out due to parasitic capacitors between the first chopper inputs in combination with attenuation resistors at the inputs. Most of these limitations, except charge injection, would vanish if the initial offset of the input amplifiers could be reduced by trimming or by autozeroing. Trimming is undesirable and not preferred in mass-production due to additional test time, cost and complexity, and lack of stability over temperature and time. One cannot simply autozero an instrumentation amplifier as was done in the prior art for OpAmps (U.S. Pat. No. 6,734,723, Huijsing et al.), because in accordance with FIGS. 1 and 2, the input voltage is not zero, but instead, the input stages carry the input and feedback voltages, respectively. In that regard, FIG. 3 presents a prior art chopper-stabilized OpAmp. Because an OpAmp is a high gain amplifier used with negative feedback, the closed loop differential input voltage to amplifier g3 is zero, so that the input to chopper Ch2 is simply the accumulated offsets of amplifiers g3, g2 and g1 as referred to the input of amplifier g3.
As used herein and in the disclosure and claims of the present invention to follow, the word stability and the various other forms of the word sometimes refer to stability in the sense of the absence of significant drift over time and temperature, not stability in the sense of absence of self oscillation or ringing, or hangup on either rail.